Nate's Duncan trade is the worst, but I want the extra 0.5M cash so I'll also mention the 2nd worst. And the 3rd through 6th worst because this is a fun question.
2. Jon selling off 2 great specs in Monroe and CJ as an overreaction to CJ's turnovers (which can be fixed over time, Dame is a prime example) to get his 2014 back and one other shitty pick is so stupid. Especially because 2014 is 3 years out and he may not be tanking anymore by then. I don't think it made any sense and is not even fixable with additions/removals, but just trading CJ or Monroe for those 2 picks would make the deal slightly better.
3. Nate giving up Tobias Harris for Jason Terry's corpse makes no sense. Harris looks like a nice prospect
4/5. While Panic's trades for mediocre bigs were abysmal, they still rank 4th and 5th worst for me because the amount of value he gave up wasn't huge.
Other than those top 5, there's a bunch of others. I don't think the Conley/MIddleton trade is that bad because the Hawks best window for another ring is this year.
6. The Kyle Lowry trade was also really bad - he is way more valuable than a single pick which will probably be in the early late teens / early 20s because both the Clips/Nugs are playoff teams this year